1. Field of the Invention
The invention relates to a vertical furnace for processing wafers. Such a treatment has to be realized on a relatively high temperature with a relatively large accuracy of the temperature in a controlled gas atmosphere.
2. Description of the Related Art
Recent requirements to processing of such wafers require a relatively high temperature. In the art vertical furnaces are known, which can either be used stand alone or in combination with a cluster. Such a furnace comprises an inner sleeve heating coils insulation and an outer sleeve. Usually the inner sleeve is made of a quartz material which bears on a support structure. In most cases this support structure is cooled.
However, at increasing the temperature in the furnace, e.g. to about 1250.degree. C. the quartz material will deglaze and degrade. Because of that radiation of the heating elements provided around the inner sleeve will not be transmitted through this sleeve so that insufficient heating of the wafers result. Furthermore, the material becomes brittle and can easily break.
Other ceramic materials are known in the art which are resistant to higher temperatures without degrading transmission properties. E.g. silicon carbide can be used to that end.
However, the heat conductivity of such materials is relatively high so that a considerable part of the heat generated in the heating zone will get lost to the support structure. Furthermore such ceramic materials being resistant to a relatively high temperature have a relatively high coefficient of expansion.